When observing a scene with an imaging system, the observer watches, in an eyepiece (or a pair of eyepieces), the image created by the active display. The space between the eye of the observer and the eyepiece 3 allows light from the display to escape, as illustrated in FIG. 1.
A first solution for eliminating these leakages of light consists in using an eyecup, usually made of flexible rubber, which fits exactly around the eye, or a mask which surrounds both eyes. FIG. 2 shows an example of a mask 10′ with which an imaging system is equipped, in this case an IR camera 1. This eyecup and this mask are called eyecup. This solution is insufficient, because it does not guarantee optical seal-tightness, notably when the observer moves the imaging system away without first switching off the display. Furthermore, there can also be leakages of light because the eyecup, flexible or rigid, is not in perfect contact all around the surround of the eye.
A second solution consists in complementing the preceding solution with an electro-optical device which is capable of detecting the presence of the eye at the right distance on each eyepiece and capable of switching on the display only when the presence of the eyes in the right place is confirmed. Such an imaging system is complicated and not necessarily compatible with camera usage conditions (dust, rain, etc.). Furthermore, this kind of electro-optical device emits light and is therefore not stealthy.
A third solution is to place, in the eyecup, a switch which switches on the display by means of a pressure of the eyecup on the observer. The drawback with this solution is that it does not guarantee that the light will not leak. In practice, the display can be switched on simply by pressing on the switch with the hands, for example.